In recent years, attention is being given to flat displays, especially to electroluminescence (hereinafter, simply referred to as “EL”) devices. The EL devices are self-emission type devices, and thus have advantages such as high visibility, wide viewing angles, and high responsivity.
The EL devices now practically used include inorganic EL devices using inorganic light-emitting materials and organic EL devices using organic light-emitting materials. Inorganic EL devices using inorganic materials such as zinc sulfide require a relatively high alternating voltage to operate, and therefore driving circuits thereof are likely to be complicated. In addition, such inorganic EL devices have a problem that brightness is low. For these reasons, inorganic EL devices have not been so actively developed for practical use.
As for organic EL devices, in 1987, Tang et al. proposed an organic EL device using an organic light-emitting material and having a two-layer structure in which a hole transport layer and an organic light-emitting layer are stacked (see, for example, Applied Physics Letters, 51, 1987, P. 913). It has been reported that this organic EL device achieved light emission having a luminance of 1,000 cd/m2 or more at a driving voltage of 10 V or less. This proposal by Tang et al. has stimulated the development of organic EL devices using organic light-emitting materials, and thereafter organic EL devices have been actively developed until now.
Hereinbelow, an organic EL device now generally studied will be described with reference to FIG. 23. An organic EL device 200 shown in FIG. 23 is formed by stacking a transparent or semi-transparent hole injection electrode 2, a hole transport layer 3, a light-emitting layer 6, and an electron injection electrode 7 on a transparent substrate 1 in order of mention. It is to be noted that the organic EL device 200 may further include a hole injection layer provided between the hole injection electrode 2 and the hole transport layer 3, and/or an electron transport layer provided between the light-emitting layer 6 and the electron injection electrode 7, and/or an electron injection layer provided between the electron injection electrode 7 and the electron transport layer.
As the hole injection electrode, a transparent conductive ITO (Indium Tin Oxide) film can be used. Such an ITO film is formed by, for example, a sputtering method, an electron beam evaporation method, or an ion plating method so that the transparency thereof can be increased or the resistivity thereof can be lowered.
Examples of a material for forming the hole transport layer include diamine derivatives used by Tang et al., such as N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD). In general, diamine derivatives are excellent in transparency, and therefore a hole transport layer formed using a diamine derivative is substantially transparent even when it has a thickness of about 80 nm.
The light-emitting layer is generally formed by vacuum-evaporating an electron transportable light-emitting material such as tris(8-quinolinolato)aluminum (Alq3) so as to have a thickness of several tens of nanometer, as has been reported by Tang et al. The organic EL device, however, may have a so-called double hetero structure composed of a relatively thin light-emitting layer and an electron transport layer having a thickness of about 20 nm stacked on the light-emitting layer, for the purpose of achieving various luminescent colors.
As the electron injection electrode, an electrode formed using an alloy composed of a metal having a low work function and a low electron injection barrier and a stable metal having a relatively high work function, such as an MgAg alloy proposed by Tang et al. or an AlLi alloy, or a laminated electrode obtained by, for example, stacking an electron injection layer formed from LiF and an aluminum layer is often used.
Further, there is known an organic EL display using low-temperature polysilicon thin film transistors (TFT) which drive individual pixels, as shown in Journal of the Society for Information Display, vol. 8, No. 2, pp. 93-97.
In the meantime, a display generally includes a circular polarizing plate. By providing a circular polarizing plate, it is possible to prevent external light which has entered the display from being reflected by the surface of a metal electrode such as an electron injection electrode formed using an MgAg alloy or the like, thereby preventing deterioration of contrast.
A display using a conventional organic EL device can be manufactured by forming a transparent hole injection electrode, an organic layer, and an electron injection electrode on a transparent substrate in order of mention. In this case, light is extracted from the hole injection electrode side. In the case of an active matrix display having thin film transistors which drive individual pixels, the thin film transistors are arranged on the transparent substrate, and therefore it is difficult for such a conventional active matrix display to have a high aperture ratio.
In order to improve the aperture ratio of such an active matrix display to achieve high brightness, a structure in which a substrate having thin film transistors thereon is provided so as to be opposed to a top surface through which light is extracted has been proposed (“Top emission structure advantageous for high brightness and high definition”, TRIGGER, vol. 10, pp. 12-13, 2001).
A top emission-type organic EL device will be described with reference to FIG. 24. An organic EL device 210 shown in FIG. 24 is formed by stacking a hole injection electrode 2, a hole transport layer 3, a light-emitting layer 6, a transparent electron injection electrode 7, and a protection layer 211 on a substrate 1 in order of mention. This organic EL device 210 is different from the organic EL device 200 shown in FIG. 23 in that the electron injection electrode needs to have transparency. In the above-described proposal of the top emission structure, a thin film of an MgAg alloy proposed by Tang et al. with a thickness of about 10 nm is used as a transparent electron injection electrode, and a transparent resin film or the like is used as a protection layer.
As described above, in a top emission-type light-emitting device, an electrode arranged on the top side needs to have transparency. Such a top emission-type light-emitting device can be manufactured by forming a thin film transistor on a back substrate which is to be arranged on the bottom side, stacking organic layers containing organic materials, such as a light-emitting layer, one after another, and then forming a transparent electrode on the top of the organic layers. However, in this case, formation of an ITO film usually used as a transparent electrode involves a problem that the organic layers provided below the ITO film are deteriorated under the influence of heat applied thereto during forming the ITO film so that carrier injection does not sufficiently occur. In the case of the top emission-type organic EL device 210 shown in FIG. 24, the electron injection electrode 7 is made transparent, and therefore a thin metal film or resin film is used for a light extraction surface. In general, an organic EL device is deteriorated under the influence of moisture or oxygen, thereby lowering brightness or increasing dark spots. In a case where a thin metal film or resin film is used for a light extraction surface, the lifespan of the device is shortened because such a film is inferior in moisture and oxygen barrier properties to a glass substrate or the like. As described above, a top emission-type light-emitting device can have a high aperture ratio, but it is difficult to achieve high brightness, high reliability, and long lifespan at the same time.
Further, it is desired that displays have high brightness and long lifespan. The brightness of a display using an organic EL device can be improved by increasing the current density flowing through an organic light-emitting material of the organic EL device, but the organic light-emitting material is likely to deteriorate due to an increased current density, thereby shortening the lifespan of the display.
On the other hand, the brightness of the display can also be improved by increasing the luminous flux from the light-emitting layer of the organic EL device. The luminous flux from the light-emitting layer can be increased by increasing the contact area between the electrode and the light-emitting layer. The contact area between the electrode and the light-emitting layer can be increased by, for example, allowing the transparent substrate of the light-emitting device to have surface irregularities or patterning the hole injection electrode with irregularities. Such a method for increasing the contact area between the electrode and the light-emitting layer can increase the surface area of the organic EL device by a factor of about 2 to 3 as compared to that of a conventional organic EL device, but cannot significantly increase the surface area of the organic EL device.
Further, each of the organic layers of the conventional organic EL device is formed as a thin film, and therefore it is necessary to control the film thickness thereof with high accuracy. If the thickness of the organic layer is not uniform, there is a case where the in-plane uniformity of luminous brightness becomes poor.
Furthermore, some conventional organic EL displays take measures against external light reflection by the use of a circular polarizing plate. However, the use of a circular polarizing plate involves a problem that the circular polarizing plate attenuates not only external light but also light emitted from the organic EL device. This also makes it difficult to achieve high brightness.